Better abstractions for splitting lengths out from VarZeroVec

[Manishearth]

Currently the VarZeroVec allocation is structured as [length, ...offsets, ...data]. However, there are reasons the length could come in externally, perhaps the length is already known because it is a map (#5376), or the length is known constant time (#5240).

We should have a VarZeroSliceUnbound type that represents [...offsets, ...data], whose functions require an externally-provided length. VZS will simply pass in its first length bytes, but other types can do fancier things.

(if not a separate type, we should at least refactor the VZV internal functions to be reusable this way)

Ideally the map polymorphism solution (#3128) will allow for a map type that externally stores a length as well. I'm not quite sure how that would work, but we can design that later.